1. Field of the Invention
The present invention relates to a wavelength conversion device and a wavelength conversion method and, more particularly, to all-optical wavelength conversion used in an optical communication system.
2. Description of the Related Art
A technology of performing signal processing on an optical signal without converting to an electric signal, that is, performing all-optical signal processing is important in an optical communication system.
Referring to FIG. 9, a description will be given of a conventional differential phase shift keying (DPSK) signal regenerator (see, for example, Masayuki MATSUMOTO, “A Fiber-based All-optical 3R Regenerator for DPSK Signals” Proceedings of the IEICE General Conference 2006, B-10-22).
A DPSK signal input into a DPSK signal regenerator 100 is divided into two: one is sent to a delay interferometer 105 and the other is sent to a clock regenerator 180.
The delay interferometer 105 converts the DPSK signal into an on/off keying (OOK) signal. The OOK signal generated in the delay interferometer 105 is sent to an all-optical wavelength converter 110.
The all-optical wavelength converter 110 performs wavelength conversion and amplitude stabilization of the optical signal. The wavelength-converted OOK signal which has been wavelength-converted in the all-optical wavelength converter 110 is sent to a phase modulator 190.
On the other hand, the clock regenerator 180 extracts clock components from the DPSK signal and generates an optical clock pulse signal. The optical clock pulse signal is sent to the phase modulator 190.
The phase modulator 190 is provided with, for example, a dispersion flat fiber (DFF) as a high non-linear fiber. The wavelength-converted OOK signal and the optical clock pulse signal are input into the dispersion flat fiber. A phase modulation pattern that matches with the intensity modulation pattern of the wavelength-converted OOK signal is superimposed on the optical clock pulse signal due to cross-phase modulation (XPM) performed during propagation through the dispersion flat fiber. As a result, a wavelength-converted DPSK signal is output from the phase modulator 190.
The all-optical wavelength converter 110 is provided with an optical amplifier 142, a dispersion flat fiber (DFF) 146 as a high non-linear fiber, and an optical band-pass filter 148. Explanation will be made on the configuration and operation of the all-optical wavelength converter 110 with reference to FIGS. 10 to 12D.
FIG. 10 is a diagram schematically illustrating the configuration of the all-optical wavelength converter. FIGS. 11 and 12 are diagrams illustrating wavelength conversion in the all-optical wavelength converter.
The optical amplifier 142 amplifies an input OOK signal (indicated by an arrow S141 in FIG. 10), and generates an amplified signal (indicated by an arrow S143 in FIG. 10) ((1) of FIG. 11). The dispersion flat fiber 146 spreads a wavelength spectrum width of the amplified signal S143, and generates a DFF signal (indicated by an arrow S147 in FIG. 10). The optical band-pass filter 148 has a wavelength band having a different center wavelength from that of the input OOK signal S141 ((2) of FIG. 11). Therefore, a converted OOK signal (indicated by an arrow S149 in FIG. 10), which is an output from the optical band-pass filter 148, is converted into an OOK signal having a wavelength different by wavelength shift amount of Δλ from that of the input OOK signal S141 ((3) of FIG. 11).
Referring to FIG. 12, the relationship between the signal intensity of the amplified signal S143 and the wavelength spectrum width of the DFF signal S147.
Assuming that a DFF signal indicated by II in (2) of FIG. 12 has been obtained by a self-phase modulation in the dispersion flat fiber 146 on an amplified signal indicated by II in (1) of FIG. 12. When the signal intensity of the amplified signal is increased (indicated by I in (1) of FIG. 12), the wavelength spectrum width of the DFF signal becomes greater (indicated by I in (2) of FIG. 12). In contrast, when the signal intensity of the amplified signal is decreased (indicated by III in (1) of FIG. 12), the wavelength spectrum width of the DFF signal becomes smaller (indicated by III in (2) of FIG. 12).
A flat wavelength spectrum as illustrated in (2) of FIG. 12 may be obtained in the dispersion flat fiber 146. By utilizing this dispersion flat fiber 146, since the intensity of the DFF signal will be substantially constant even there are fluctuations in intensity of the input signal, an influence of the fluctuations in intensity of the input signal can be suppressed and a noise component can be removed.
A time waveform of the amplified signal is illustrated in (3) of FIG. 12. Further, a time waveform of the wavelength-converted OOK signal S149, which is the output from the wavelength converter, is illustrated in (4) of FIG. 12. A noise component indicated by IV in (4) of FIG. 12 is not included in the time waveform of the wavelength-converted OOK signal S149 (see (4) of FIG. 12).
With these characteristics, the all-wavelength converter functions as an identifying circuit as well as the wavelength converter.
However, when a high-speed optical signal having a data rate of 40 Gbps or more is converted using the dispersion flat fiber, there is a tendency that a waveform shaping function is markedly degraded when the wavelength conversion amount is large.
In view of the above, in the DPSK signal regenerator disclosed in the document referred above, wavelength converters are connected in multiple stages, and at each of the wavelength converters, the wavelength conversion amount is adjusted to an extent such that the waveform shaping function is not degraded. Specifically, wavelength converters are connected in five stages in order to achieve wavelength conversion of 10 nm, wherein the wavelength shift amount in each of the wavelength converters is set to 2 nm.
Therefore, the DPSK signal regenerator is difficult in miniaturization, and further, is economically disadvantageous.
Moreover, if the data rate is increased, it is necessary to narrow the width of an optical pulse in proportion to a transmission rate. In this case, it is also necessary to decrease a dispersion value of the dispersion flat fiber. An appropriate dispersion value required for the dispersion flat fiber is proportional to the square of a pulse width, that is, is inversely proportional to the square of the data rate. As a consequence, if the data rate is increased four times from 40 Gbps to 160 Gbps, a dispersion value required for the dispersion flat fiber will be 1/16. This signifies that a dispersion value of −0.03 ps/nm/km, having a significantly small absolute value with respect to a dispersion value of −0.5 ps/nm/km in the fiber disclosed in the above-referred document, is required. However, fabrication of a dispersion flat fiber having a dispersion value having such small absolute value is difficult.